1. Field of the Invention
The present invention relates to a small zoom lens and an imaging apparatus which are preferably used for a video camera, a digital still camera and the like.
2. Description of Related Art
As a zoom lens for use in a video camera, a digital still camera and the like, for example, known is a three-group zoom lens in which the magnification is varied by moving lens groups along an optical axis so as to change spaces between the respective lens groups. In recent years, in imaging optical systems like this, there has been an increasing demand for optical systems which are smaller in size and are able to form images with higher quality. JP-A-2003-149556, JP-A-2006-39523, JP-A-2006-84829 and JP-A-2006-23678 propose techniques in which the whole optical system of a three-group zoom lens is made smaller in size while being configured to form images with higher quality by arranging refractive powers of the lens groups in a negative-positive-positive configuration and making effective use of aspherical surfaces and cemented lenses.
Here, in the three-group zoom lens, in order to attain the miniaturization, a reduction in maximum length of the lens when in use and a reduction in overall thickness of each lens group are necessary. In addition, in order to attain the formation of images with higher quality, spherical aberration, field curvature and chromatic aberration need to be corrected. In the zoom lenses described in the patent publications above, however, since the focal length of the whole of a first lens group is made longer or the thickness of the whole of the first lens group is made thicker, the overall length of the zoom lens tends to be long, and the miniaturization has not yet been attained sufficiently. In addition, in the event that a further miniaturization is attempted to attained based on the configuration of the zoom lens described in each of the aforesaid patent documents, the formation of images with higher quality cannot be realized.
In addition, in JP-A-2003-149556, since a surface of the second lens group which lies closest to the object is made aspherical and this aspherical lens is made to constitute a cemented lens, the aspherical surface cannot be provided only on one surface, leading to an issue that among aberrations, in particular, spherical aberration cannot be corrected sufficiently. In addition, in the configuration of JP-A-2006-39523, since a second lens group is made up of a cemented lens which is made up by cementing three lenses together, the degree of freedom in design is reduced, and the correction of, in particular, spherical aberration is made difficult. In addition, in the configuration of JP-A-2006-84829, since the focal length of a second lens group which constitutes a primary variable power or focal length varying portion is long, the overall length of the zoom lens becomes long, whereby the miniaturization is made difficult to be realized. In addition, in the configuration of JP-A-2006-23678, since a second lens group is made up of two lenses; a positive lens and a negative lens, the correction of on-axis chromatic aberration is made difficult. Consequently, the realization of a zoom lens system has long been desired which is smaller in size and is able to form images with higher quality, compared to the three-group zoom lenses according to the related art.